Glossary for Postural Orthostatic Tachycardia Syndrome

GlossaryAdrenergic receptors: there are four basic types of adrenergic receptors that can be bound by norepinephrine and epinephrine. These are alpha-1, alpha-2, beta-1, and beta-2 adrenergic receptors.

Agonist: drug that binds and activates a receptor in a way the mimics the endogenous substance, e.g. norepinephrine

Alpha-1 adrenergic receptors: found on smooth muscle (involuntary) of arteries, veins, and sphincters of the urinary and gastrointestinal tract. Medications that activate these receptors will cause constriction of the blood vessels.

Alpha-2 adrenergic receptors: located on cells of the adrenal gland. When stimulated, these receptors activate a negative feedback system that decreases the release of norepinephrine by the sympathetic nervous system.

Alpha-adrenergic drugs: produce contraction of smooth muscle and constriction of blood vessels by stimulating the alpha-1 adrenergic receptors. These are sympathomimetic drugs that mimic norepinephrine. Midodrine is an example of this group taken by people with POTS, and works to increase blood pressure.

Antagonist: drug that binds and blocks a receptor without activating it

Antibodies: proteins made by our immune system to help us fight off bacteria, viruses, fungi and other infections that are in the blood or extracellular fluid (outside of our body cells). These help us to manage the invaders and eventually feel better as the infection recedes.

Autoantibodies: antibodies made by the immune system that have mistakenly targeted our own body cells. They can damage healthy tissues and interfere with their normal functioning.

Autonomic Nervous System: the automatic portion of the peripheral nervous system that controls heart rate, blood pressure, movement of food through digestive tract, pupil diameter, diameter of the bronchi in/bronchioles in the lungs, etc. It is divided into the sympathetic (fight or flight) and parasympathetic (relaxation) nervous systems.

Beta-1 adrenergic receptors: found mainly in the heart, these receptors increase the heart rate and force of contraction when activated by norepinephrine, epinephrine, or medication

Beta-2 adrenergic receptors: found mainly on the smooth muscle. When these receptors are activated, they cause the blood vessels of the skeletal muscles and coronary arteries to dilate

Beta-adrenergic drugs: these both stimulate the heart (beta-1 receptors) to increase heart rate and force of contraction and cause smooth muscle relaxation (beta-2 receptors) which dilates the blood vessels. Sympathomimetic drugs.

Beta blockers: medications that block the beta-adrenergic receptors for norepinephrine. They prevent norepinephrine from binding the receptor, and decrease the impact of the sympathetic nervous system. Classical use is to reduce the heart rate or force of contraction in the treatment of angina, hypertension, or arrhythmias, but these are often used for POTS as well.

Cholinesterase inhibitors: inhibit the activity of the enzyme acetylcholinesterase. Acetylcholinesterase normally breaks the neurotransmitter acetylcholine (found in the parasympathetic nervous system and neuromuscular junction), thereby inactivating it. When cholinesterase inhibitors are given, this enzyme is inhibited, and levels of acetylcholine in the synapse increase.

Dysautonomia: a group of disorders all characterized by the poor functioning of the autonomic nervous system. Dysautonomia includes POTS, neurocardiogenic syncope, pure autonomic failure, autoimmune autonomic ganglionopathy, and multiple system atrophy among others.

Ehler's-Danlos Syndrome (EDS): Ehlers-Danlos syndrome is a group of disorders that affect connective tissues, which are tissues that support the skin, bones, blood vessels, and other organs. Defects in connective tissues cause the signs and symptoms of Ehlers-Danlos syndrome, which vary from mildly loose joints to life-threatening complications.

Electrophysiologist (EP): a cardiologist who specializes in the electrical activity and electrical pathways that run through the heart.

Exercise Intolerance: the person is unable to exercise at the level or duration that would be expected for someone of their level of general health. It can be characterized by unusually severe pain, fatigue, nausea, dizziness, or other side effects after completion of exercise.

Gastroparesis (GP): a condition in which the spontaneous movement of the muscles (motility) in your stomach does not function normally.  This prevents your stomach from emptying properly and can interfere with normal digestion, cause nausea and vomiting, and cause problems with blood sugar levels and nutrition.

Hypertension: high blood pressure defined as >140/90 mmHg on three separate readings

Hypokalemia: low blood potassium levels which can occur when taking Florinef. This drug acts like aldosterone to make the kidneys retain sodium and excrete potassium. If the potassium drops low enough, it can cause muscle weakness and pain, lethary, and eventually be fatal. Taking Thermotabs or another potassium supplement when on Florinef can prevent this condition.

Hypotension: low blood pressure. It is usually defined as less than 90/60 mmHg.

Hypovolemia: low circulating blood volume

Noradregeneric receptors: norepinephrine receptors that are bound by norepinephrine and/or epinephrine when released by the sympathetic nervous system

Norepinephrine: the major neurotransmitter of the sympathetic (fight or flight) nervous system. It can cause constriction of blood vessels and increased blood pressure.

Orthostatic: related to erect postural; standing upright

Orthostatic intolerance: body’s inability to deal with changes in gravity as you move from laying/sitting to standing

Parasympathetic Nervous System: the part of the autonomic nervous system that increases relaxation. It promotes digestion, slows the heart and respiratory rates, and decreases blood pressure. It is mediated by the neurotransmitter acetylcholine.

Poor Man's Tilt Table Test: This quick check of heart rate can be done in any doctor's office or at home to see if orthostatic intolerance is an issue.  Have the person lay flat for 2 minutes and take the heart rate.  Repeat the heart rate count after 2 minutes of sitting and again after 2 minutes of standing still.

Post-prandial pain: abdominal pain that occurs after eating

QSART (Quantitative Sudomotor Autonomic Reflex Testing): Test used to measure the amount of sweat produced when given mild electrical stimulation. In POTS, sometimes the nerves controlling this reflex are damaged, and this can be determined by lack of sweating during the test. High sweat production could mean overactivity of the sympathetic nervous system.

Spoon Theory: used to explain the rationing of energy for people with a variety of invisible illnesses.  Each spoon represents a certain amount of energy.  If you have 12 spoons and spend two to get dressed, one to eat breakfast, and one to brush your teeth, then you only have 8 spoons left for the rest of the day. 

Sympathetic Nervous System: the “fight or flight” part of the autonomic nervous system. The sympathetic nervous system will increase heart rate and force of contraction, increase blood pressure, increase respiratory rate, and slow digestive functions. It is mediated by the neurotransmitter norepinephrine.

Sympatholytic drugs: alpha and beta adrenergic medications that block the effects of the sympathetic nervous system. They antagonize the sympathetic nervous system to decrease its activity.

Sympathomimetic drugs: alpha and beta adrenergic medications that mimic the effects of the sympathetic nervous system (fight or flight). They are agonists for the sympathetic nervous system to increase its activity.

Syndrome: a cluster of symptoms that characterize a specific disorder

Tachycardia: Excessively rapid heartbeat at rest. Typically defined as more than 100 beats per minute when at rest.

Tilt Table: Used to evaluate the cause of unexplained fainting. Initially the table is horizontal and eventually is raised to 70-80 degrees (almost upright standing) for approximately 10 minutes while they monitor your heart rate and blood pressure.

Vasoconstrictors: decrease the diameter of a blood vessel. In veins, this promotes movement of blood back toward the heart and can diminish blood pooling in the lower extremities.

Numbers to know

Normal Heart Rate

  •           Bradycardia
  •           Tachycardia

60-100 beats per minute at rest

< 60 beats per minute

>100 beats per minute


Normal blood pressure

  •            Hypotension
  •           Hypertension

<120/75 mmHg

<90/60 mmHg

>140/90 mmHg


            1 liter

           1 gram table salt

33.8 fluid ounces

388 mg sodium